The realisation that the plane was an Allied aircraft piloted by the hero only dawned on Sgt McCormack the following day when he and his crew were debriefed.

Racked with guilt, Sgt McCormack told nobody about what happened that night - but left a taped confession of the incident, which he gave to his wife before he died in 1992.

The tape has been uncovered by World War Two researcher James Cutler, who says he is "100 per cent satisfied that Guy Gibson was killed by friendly fire."

Mr Cutler has also unearthed two previously-classified reports in the National Archives that contradict official RAF records stating that no planes were encountered on the night Wing Commander Gibson was killed.

One of them is the combat report from the crew of Sgt McCormack's Lancaster, which describes the attack on what they thought was the Junkers 88.

The second report is from the crew of another Lancaster, who noted 'plane shot down - as aircraft burst on ground a red light resembling Target Indicator seen.'

The location given was over Steenbergen. Gibson's plane had been carrying red Target Indicator flares which he was unable to drop over Germany due to a fault.

Mr Cutler said: "It could only have been Guy Gibson's plane because the co-ordinates in both these new documents were right over the spot where his plane came down.

"I am satisfied 100 per cent that Guy Gibson was killed by friendly fire and 99.9 per cent sure that he was shot down by Bernard McCormack's plane.

"For Guy Gibson to be killed by friendly fire was a huge blunder."

Wing Commander Gibson was awarded the Victoria Cross in 1943 for successfully leading 617 Squadron on the daring raids on a series of damns in the industrial Rhur valley in Germany.

On the night of September 19, 1944, Gibson led 227 Lancaster bombers and 10 Mosquitos on an attack of Rheydt and Muchengladbach in Germany

They faced little opposition from enemy planes and were returning over the Netherlands when Gibson's plane came down.

In his confession tape, Sgt McCormack can be heard saying: "We were on the way back over Holland and then all of a sudden this kite comes right behind us twin engines and a single rudder - and it comes bouncing in towards us so we opened fire and we blew him up.

"When we got back we claimed a Ju 88 show down. The following day we were called in to the office and we were quizzed again.

"(RAF Intelligence Office) 'What made you think it was a Ju 88?' We said 'it had twin engines and a single rudder.' He said: 'So has a Mosquito.'

"Well supposing - he put it very nicely - he said, 'supposing a Mosquito - his radio and his radar was knocked out an he was lost and he spotted a Lancaster - he would only want to follow it home wouldn't he? And it turned out it was 'Gibbo' we shot down."

Sgt McCormack, who went on to become the mayor of Holyhead in north Wales, died in 1992.